The present invention relates to a process for reducing the carbon content of aqueous hydrogen peroxide solutions. The process is directed in particular at aqueous hydrogen peroxide solutions which are obtained within the context of the so-called anthraquinone process for preparing hydrogen peroxide.
Hydrogen peroxide may be produced in a variety of ways, but the anthraquinone cyclic process is the most significant from an industrial point of view. As shown in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A13, 5th ed. (1989), pages 447-457, this cyclic process includes (a) hydrogenation of an alkylanthraquinone in an organic solvent or solvent mixture which is not miscible, or only slightly miscible, with water, (b) oxidation of the alkylanthrahydroquinone which is formed with the formation of hydrogen peroxide and re-formation of the alkylanthraquinone, (c) extraction of the hydrogen peroxide with water and (d) various steps for purifying the aqueous H.sub.2 O.sub.2 solutions which depend on the desired quality of the H.sub.2 O.sub.2.
The H.sub.2 O.sub.2 solution obtained following extraction contains volatile and non-volatile carbon compounds, depending on the particular process. In order to fulfill quality requirements, various methods for reducing the carbon content (C-content) have been disclosed. The C-content of the aqueous H.sub.2 O.sub.2 extract can be reduced, for instance, by using extractive measures (see for example GB 841 323). However, this type of method is very expensive and the residual C-content thereby achieved does not in general correspond to the requirements demanded for high quality H.sub.2 O.sub.2 solutions.
A simplified and improved method is disclosed in U.S. Pat. No. 4,759,921 wherein the residual C-content of an aqueous H.sub.2 O.sub.2 solution is lowered by 100 to 120 mg/l by phase separation of a mixture of the H.sub.2 O.sub.2 extract and 0.5 to 5% by weight of a quinone solvent using a special coalescer. Although some volatile C-compounds can be separated out during concentration of the H.sub.2 O.sub.2 extract, the product still contains too high a residual C-content for many purposes.
A method for lowering the carbon content of aqueous H.sub.2 O.sub.2 solutions is known from U.S. Pat. No. 2,919,975 which includes distillative concentration, treatment of the previously diluted concentrate with activated carbon at a temperature of less than 10.degree. C., and isolation of the activated carbon loaded with adsorbed carbon compounds. The disadvantages of this method include the following: only selected types of activated carbon, optionally pre-treated in a particular way, can be used; the carbon depleted H.sub.2 O.sub.2 solution sometimes requires an additional subsequent concentration step; the adsorption treatment is energy-consuming due to the cooling which is required; and the loaded activated carbon cannot be regenerated or can only be regenerated by using complicated procedures. A further development of the adsorption of carbon compounds contained in hydrogen peroxide solutions onto activated carbon is disclosed in U.S. Pat. No. 4,985,228 but the disadvantages pointed out above have not been overcome.
According to the process in U.S. Pat. No. 3,305,314, the H.sub.2 O.sub.2 solution is treated with an anion exchanger, preferably a strongly basic ion exchanger with quaternary ammonium groups in the form of a salt of an acid which is weaker than acetic acid, in order to reduce the C-content of aqueous H.sub.2 O.sub.2 solutions. Although anionic carbon compounds can be removed using this process, hardly any neutral and no cationic carbon compounds are removed. The use of an ion exchanger is not without problems from the safety aspect and regeneration involves very complex technical procedures in order to satisfy official limitations placed on effluents.
It is also known that the carbon content can be reduced by using organic adsorber resins with a macro-porous structure (see ES-A 547 571/6). The disadvantage of these resins is in particular the considerable cost of regeneration. After rinsing the resin with water, they have to be treated with an organic solvent and then the solvent has to be displaced again by water. Apart from solvent losses, there is also the production of an effluent which has to be disposed of.
As explained above, the previously known methods for reducing the C-content of aqueous H.sub.2 O.sub.2 solutions are still not completely satisfactory, so there is an interest in an improved and/or new type of process.